1. Field of the Invention
The present invention relates to a porous metal bond grinder or whetstone used to grind various materials and a method of manufacturing the grinder. More particularly, the porous grinder of the present invention has an increased occupancy rate of pores so that the grinding function by grinding particles is enhanced to improve the grinding quality thereof.
2. Prior Art
A grinder is used to grind various works. A grinder used for grinding is composed of grinding particles and binder and has innumerable pores formed therein. The grinding particles function as edges for cutting or grinding various works, and the binder functions as a support for combining the grinding particles with each other. Further, a large number of continuous pores function as chip pockets for discharging chips cut off by grinding particles.
Recently, various devices or apparatuses often use material such as ceramics, cemented carbide or hard metal or superspeed steel which is difficult to grind. Accordingly, the need for grinding a material of this type has increased more and more. As a grinder or a grinding wheel for grinding a material of this type, one using super grinding particles such as diamond grinding particles or boron nitride grinding particles of the cubic system is being gradually used.
As a grinder using such super grinding particles, there are various kinds of grinders such as a vitrified bond system, a resinoid bond system, a metal bond system, a silicate bond system and a rubber bond system in accordance with the kinds of the binder. These grinders have both merits and demerits, while the grinder of the metal bond system using metal and its alloy as the binder thereof is mainly used in view of its strength and long life.
The grinder of the metal bond system is manufactured by putting metal powder having grinding particles scattered uniformly into a mold together with a metal base and subjecting it to pressing and sintering (or hot pressing) processes. The binder of metal used in the metal bond grinder uses, for example, a Cu-Sn system, a Cu-Sn-Co system, a Cu-Sn-Fe-Co system, a Cu-Sn-Ni system or a Cu-Sn-Fe-Ni system or any of these systems to which phosphorus (P) is added.
The grinding particles of a conventional metal bond grinder has an extremely strong combination or binding strength as compared with the resinoid bond grinder and the vitrified bond grinder. Accordingly, the metal bond grinder can advantageously exert a sufficient retention force required to perform strong grinding by means of super grinding particles. However, the metal binder does not have sufficiently large pores to help discharge chips that are cut off by the grinding particles. Thus, "escape bores" in which chips enter are restricted to minute gaps between the metal bond grinder and a work or to minute gaps constituted by portions in the metal bond grinder in which grinding particles have fallen off. Further, in the metal bond grinder, the binding force of the grinding particles is extremely strong and accordingly when the grinding particles are worn, the worn particles have difficulty falling off from the binder. Hence, it is also difficult to form the "escape bores" that are formed by the grinding particles that have fallen off.
As described above, in the conventional metal bond grinder, the discharging of chips is deteriorated and loading occurs easily. Accordingly, the grinding resistance increases and the grinding quality deteriorates, so that the heat generated is increased. Further, the grinder has a tendency to unsuccessfully finish the surface of a work. Accordingly, it is very difficult to increase the contact area of the grinder and the work and perform grinding with higher efficiency.
In addition, the conventional metal bond grinder has a low sintering temperature and is hence apt to be softened at a low temperature. Accordingly, there is a defect in that plastic deformation occurs due to the heat generated upon grinding and the loading takes place in the surface of the grinder.
In order to eliminate the above defects, for example, Japanese Patent Application Laid-Open No. 59(1984)-182064 discloses a continuous porous metal bond grinder. However, this metal bond grinder does not utilize the powder sintering method. More particularly, in the manufacturing method of this metal bond grinder, an inorganic compound that is melted by solvent is sintered into a desired shape. Thereafter, gaps or spaces of the sintered body are filled with grinding particles and the body having spaces filled with grinding particles is heated. Then, melted metal or alloy is pressed into the spaces of the thus sintered body filled with the grinding particles and is then solidified. Thereafter, the inorganic compound is liquated out by a solvent.
Further, Japanese Patent Application Publication No. 54(1979)-31727 discloses a grinder that has many layers of metal coatings formed thereon and is sintered so as to be structured like a vitrified bond by hot press and has pores. In addition to this, various measures for preventing reduction in grinding quality have been proposed.
Furthermore, Japanese Patent Application Laid-Open No. 3(1991)-264263 discloses a grinder using cast iron for the purpose of preventing loading of the grinder. The grinder using the cast iron as a bond advantageously has great strength and high rigidity and is worn in the brittle fracture manner without the occurrence of plastic deformation, so that loading is less likely to occur. However, the bond of this grinder is too strong and accordingly the dressing property is deteriorated as compared with the bond of the copper system.
By forming a large number of pores within the grinder, grinding liquid can be impregnated into the pores to enhance the cooling characteristics of the grinder and a large number of chip pockets can be formed in the grinding surface to improve the discharging characteristics of chips. Further, the grinding resistance can be made small by the pores to improve the grinding quality. In other words, it can be expected that less heat is generated and the surface of a work is finished with high quality. However, when a large number of pores are formed in the conventional copper system metal bond grinder, the strength and the retention force of grinding particles thereof are naturally reduced, so that the sufficient grinding performance cannot be obtained.
Further, in the grinder using non-porous cast iron as the bond, iron powder is added to cast iron powder because of the inferiority of the sintering characteristics of the cast iron powder and then pressurized with the load of 8,000 kgf/cm.sup.2 to 1,000 kgf/cm.sup.2. By adding the iron powder, the original brittle fracture characteristic of the cast iron is lost and plastic deformation is apt to occur due to heat generated upon grinding in the same manner as the copper system bond, so that the characteristics of the cast iron cannot be drawn out sufficiently.